The Manufacturing Stained Glass

Medieval Techniques

Pot-metal glass:
Theophilus, a German monk writing in the 12th Century, recorded the method of creating the stained glass of the time. Sand, potash and lime were melted together in clay pots and coloured by adding metallic oxides: copper for red, iron for green, cobalt for blue and so on.
Pot-metal glass on its own, particularly red glass, was too dim to let in much light. Glass was 'flashed' by dipping white glass on the blowpipe into a pot of red glass and then blowing. This provided sheets of glass with a thin layer of colour, which improved its translucency. It has the added advantage that parts of the coloured layer could be grounding down by an abrasive wheel; giving two colours, green and white for instance, on a single piece of glass.

Creating a window:
As paper was scarce and parchment very expensive, the
A full scale outline of a design was sketched in detail on a work top painted white (paper and parchment were both hard to come by and very expensive). The designer would specify the outlines of his drawing, and the shape and colour of the individual pieces of glass to be used, together with the position of the strips of lead (the ‘calmes’) that would join the glass.
Lead is the material of choice as it is strong and long lasting, but above all because it is sufficiently malleable to mould into shape.
Glass was cut with a 'grozing iron' and each piece laid in its position on the work top. An iron oxide pigment was used to fill in details that had first been drawn on the plan below, such as faces, torsos, clothes etc. When painted, the pieces were fired in a small furnace to fuse the paint to of the glass. They were then re-laid on the table and assembled by the glazier. The glazer used strips of grooved lead that allowed the glass to be slotted together on each side. The joints in the strips were then painstakingly soldered, and an oily cement rubbed into all the joins making them watertight. Each glass panel was then hoisted into place in the window frames and fixed with iron bars set into the supporting masonry.

A further range of colours varying from a pale lemon to a deep orange could be achieved through the discovery of 'Around 1540, a silver compound was discover, known as ‘silver stain' ,which was painted on the back of the glass and then fired in the usual way to produce pale yellows through to rich oranges, or mixed with blue stains to produce hues of green. By the mid sixteenth century a range of coloured enamels were being used to stain glass in this way. Consequently, windows began to be painted upright, like pictures, onto clear glass. Shaped glass using lead calmes were no longer required to produce a pictures, and painted glass was easier, cheaper and more detailed to produce than the old style, and became widely used until early nineteenth centuries. However, the Medieval technique re-emerged during the Gothic Revival of the Victorian times and has replaced the glass-painting method.